A new form of superstructure with a a X 2 V 5 x 1 supercell oriented at 45" to the original unit-cell axes has been found in a YBa2Cu30,_, single crystal by means of electron diffraction and electron microscopy. It is suggested that this superstructure results from a substructure modulation.
A structural instability shown in the form of a phase-transition-like internal friction peak and step-like change of lattice parameters appears at 135 K in a Tl2Ca2Ba2Cu3Ox sample with zero resistance at 115 K. It was confirmed that the structural instability at 135 K coexisted with the superconducting transition at 115 K. In addition, a small internal friction peak and an ultrasonic attenuation peak, which may be related to the lower temperature superconductive phase, were observed at 90 K in the Tl-Ca-Ba-Cu-O material.
In the single crystal BiSr ( Ca 0.35 Sr 0.15 Bi 0.40)2 Cu 2 O y a characteristic defect-superdislocation has been observed by TEM. The twist boundary resulted from the interaction of two sets of mutually perpendicular superdislocations has been analysed.
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